x86/cpu: Create helper function to parse the 'clearcpuid=' boot parameter

This is in preparation for a later commit that will reuse this code, to
make review convenient.

Factor out a helper function which does the full handling for this arg
including printing info to the console.

No functional change intended.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241220-force-cpu-bug-v2-1-7dc71bce742a@google.com